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7/27/2019 Pump Maint Guide.doc
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A Working Man’s Guide toServicing
Centrifugal PumpsMaintenance TipsA pump is a wonderful thing when operating properly. You never give it a second thought. But
when it’s not running properly, you hear about it right away. People on the receiving end are not
getting heat, cooling, process fluids, etc., and they’re quick to let you know, usually very loudly.
So here are some service procedures for centrifugal pumps that will help keep your customers in
hot water and you out of it.
But before we launch into the actual service, a few words about safety. After all, being rushed tothe emergency ward doesn’t make the job go any quicker.When working on any type of equipment, safety should always be your first concern, and
centrifugal pumps are no exception. You need to follow the proper procedures set-up at your
shop. There are several areas with which to be particularly concerned.
First, make sure the pump can be taken out of service. It may be providing services that are
extremely important. Removing a pump from service that’s providing chilled water to the
operating suite at a hospital without permission, for example, will not earn you that bonus you
deserve.
Figure1 is a typical parts diagramfor centrifugalpump.Suchdiagramscan oftenbe
foundon a manufacturer'swebsite alongwith other valuabletechnicalinformation.
Before you even start, you’ll want to remove power to the equipment. All shops have some type
of lock-out/tag-out procedure. Follow them completely; all the way through tag removal. This is
absolutely necessary, because if you occasionally forget to remove your tags, someone else willget in the habit of removing them for you, usually while you’re still working on the equipment.
Make sure you remove all sources of power. Some motors, such as at hospitals, also have anemergency power source. Isolate that source from the pump too. More will be mentioned as wego through the servicing procedures.We’re not quite ready yet to start disassembling our pump. To ensure that the whole process runs
smoothly, let’s take a few steps before proceeding. It would be nice to have an exploded diagram
of the pump before we begin. One reason for this is so we’ll know where to put all the extra parts
we have left over when we’re done. They have to go somewhere. Manufacturers don’t usually
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add parts that don’t have any function. An exploded diagram like the one shown in Figure 1 is
especially helpful. It not only illustrates the pump, it also gives us the proper part names.
Therefore, if we need to order a replacement, we know what to call it besides a “thing- muh-jig.”
Points of Service
We’re going to look at the three main areas for service. They are:• Lubrication
• Seal replacement
• Alignment.
Lubrication is probably the single most important step in a maintenance program to keep a pump
running without problems. Obviously, the manufacturer’s recommendations as to the type of
lubricant and proper lubrication intervals are the best place to start. It’s a starting point; not the
final word. The reason I say that is the manufacturer doesn’t know how his equipment is being
used. How it’s used will affect how much you lubricate it. A couple of factors that affect the
lubrication schedule are how often you run, or don’t run the pump. What type of environment is it
in? Is it hot, dirty, and dusty? Or is the pump lucky enough to reside in a cool, dry, location? You
can see that the answer to, “How often should I lubricate a pump?” is, “it all depends.” Not much
of an answer. So here’s where experience comes into play. The more you know about the
system, the better you’re able to protect it. If you’re lacking in experience, don’t worry, you have
something else to rely on: common sense.
To help you determine a lubrication schedule, check your records to see how the system is
performing. What have you done in the past to maintain the system? Where have you been
successful, or unsuccessful in the past? If you don’t know, you don’t have a good set of records.
So now, you need to start a log. There are a few things you want to put in that log: suction
pressure, discharge pressure, how the pump is running (Hot? Noisy?), when was it lubricated?
You’re going to lubricate the bearings on occasion. Generally there are two types of bearings:
sleeve bearings and ball bearings. Everything about them is different. One of the best ways to
determine when to lubricate either type is based on how hot they’re running. If you have a way to
measure the temperature, great. If not, you’ll want to err on the side of lubricating them too often.
After all, the cost of grease or oil is small compared to the time and cost of replacing the bearings.
Let’s start with the ball bearings. Before you start pumping grease into the grease fitting, check tosee if there’s a plug in the grease relief hole. If there is, remove it. You don’t want to pump new
grease into the bearings with no way of removing the old grease. Too much grease can be just as
harmful as too little grease. You want to keep pumping new grease in until you start to see all the
old grease come out.
You’re done greasing that bearing, but do you want to put the grease plug back in? Many people
will simply throw the relief hole plug away for fear the next person will forget to remove it before
they start to pump in grease. In fact, some manufacturers won’t even thread the hole to keep the
plug from getting jammed in the hole.
For sleeve bearings you’re going to want to add oil. Not just any oil, mind you. Once again, use
what the manufacturer recommends. This will be a non-detergent oil. There’s an important reason
for this. Oil evaporates - detergent doesn’t. As the oil evaporates, the concentration of the
detergent becomes stronger. You can see that each time you replenish the oil, you add evenmore detergent to the oil. Once the oil is added, it is absorbed by a wick that deposits it where it’s
needed, between the sleeve bearing and the shaft to maintain a thin film of oil between the two.
The wick has an important job. Therefore, you’ll want to inspect it. If it’s scored or burnt, replace
it. If it feels waxy, it can no longer do its job effectively.
The most likely reason for disassembling a pump is to replace the seals. Even if seal replacement
isn’t your reason for disassembly, it’s a good idea to replace the seal while you have the pump
apart. That seal kit you bought contains all the parts you need. Replace the whole thing and not
just an individual part. When you remove the seal, put on your detective’s hat. Ask yourself why
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the seal wore the way it did. Were there grooves in the seal? This could be a sign of a high
concentration of suspended or dissolved solids. Suspended solids less than 10 microns will get
between the two seal faces. Maybe you need to choose a seal that is more tolerant of these
solids. Maybe you need to determine if there’s a way to remove these solids from the system.
Your chemical concentration may be too high. A seal has two highly polished surfaces mated up
to each other. Sure, no liquid is suppose to leak between the two surfaces, but it does -it has to.
The thin film of liquid between the two surfaces heats up due to friction. The heat is then removedwhen the liquid evaporates. You never see that evaporation because it’s extremely small. But,
like the oil in the sleeve bearing, the chemicals are left behind. They can then cause grooving in
the seal.
If you find that you’re replacing seals often, perhaps you need to look at the type of seal you’re
using. If you can’t remove the high concentration of chemicals, perhaps you need a seal that is
more tolerant of the high concentration. Other factors that will affect seal selection are pH levels
and temperature. The last word on seals is that they’re most often damaged during their
installation. Be especially careful when putting the seal in. Don’t touch its surface with dirty hands.
Use soapy water to slide the seal on the shaft, since petroleum products may affect the seal. Try
not to chip the seal when placing it on the shaft. And don’t run the seal dry. Open the service
valves to flood the pump cavity before running the pump. As you disassemble the pump, you’ll
have to deal with gasket surfaces. Whenever you’re dealing with a gasket surface, be careful notto scratch these machined surfaces. It may cause a leak path. And always use new gaskets when
putting the pump back together.
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Keep It StraightAlignment is a concern for flexibly coupled pumps. The coupler’s job is to transmit power from the
motor shaft to the pump shaft. It can also take up minor misalignment. The key word here is
minor. Exactly how much has been determined by the coupler manufacturer based on several
factors, so use their recommendations.
How you check alignment is basically determined by rpm. The lower the rpm, the lesssophisticated a method you can use. At 1,750 rpm you can get away with aligning the shafts with
just a straight edge and taper gauges. At 3,500 you’ll want to use a dial indicator. For higher rpm
and greater accuracy, you may need a laser.
Check that the pump and motor shafts rotate around the same axis. This means you have to
check the parallel and angular alignment as in figures 2 and 3. To change these alignments, you’ll
have to place or remove shims from under the motor and move the motor from side to side. After
you’ve placed shims under the motor, tighten the motor down to check alignment again. It’s
always better to use fewer, larger shims than a larger number of thin shims. After you’ve placed
the pump in service, recheck the alignment when the pump reaches its operating temperature.
Obviously, there’s a lot to cover when servicing a centrifugal pump. Here, we’ve just touched on
some of the main areas of concern. Along with that, my final advice is to use common sense and
be methodical when working so as to not damage the pump - or yourself.
This article is an abridged version of a Little Red Schoolhouse® Staff article that originally appeared in the March, 1999 issue of Contracting Business.